The present invention relates to a cover for closing a top opening of defined inside dimensions of a thermite reaction chamber of a crucible, the cover including for this purpose a bottom peripheral edge suitable for bearing against a top peripheral edge defining said top opening.
Such a cover has been proposed by the Applicant in its European patent EP 0 407 240 B1, in the context of a complete apparatus for thermite welding, the apparatus also comprising a crucible for producing metal in the liquid state by the thermite reaction, and a mold for receiving and confining said molten metal in a space between two metal parts that are to be welded together by means of said metal.
In the embodiment described in EP 0 407 240 B1, the bottom edge of the cover bears against the top edge of the crucible in a manner that is substantially continuous and substantially leaktight, and the cover is provided with vent slots opening out into a central depression of a top wall of the cover and allowing the fumes given off by the thermite reaction to escape to the atmosphere, while trapping a maximum number of particles that might be projected by the reaction.
In order to enable the crucible to be used under certain conditions in which the fumes could be harmful or even dangerous, for example in a tunnel or underground, EP 0 407 240 B1 proposes placing a filter cap on the cover, said cap covering at least the vent slots.
Such a filter cap is sold by the Applicant in a shape that is slightly different from the shape described in EP 0 407 240 B1, while nevertheless being of the same general design as that described, and when it is associated with a cover presenting the shape as described in EP 0 407 240 B1 it is recognized as being effective.
Nevertheless, it has been felt that the fume filtering technique described in EP 0 407 240 B1 could be improved, and that is the object of the present invention.
As presently manufactured and sold by the Applicant, the filter cap is in the form of a steel frustoconical chimney that flares from a bottom edge making substantially continuous leaktight contact with a peripheral rim of the crucible cover towards a top edge for releasably and interchangeably receiving a disk of refractory felt that acts as a filter between a bottom support grid secured at its periphery to the top edge of the chimney and a top retention grid that is releasably held on the chimney by spring clips.
When the welding apparatus is in use, which apparatus is in all other points identical to that described by way of preferred example in EP 0 407 240 B1, the filter cap fitted with a new refractory felt is placed on the rim of the crucible cover while the cover is separate from the crucible, and the crucible is placed on the mold, after which the thermite charge in the crucible chamber is ignited and the assembly constituted by the cover and the filter cap is placed thereon. Once the thermite reaction has terminated, the filter cap is removed from the cover and then the mold is separated from the crucible and its cover by destroying them (if they are made for single use as described in EP 0 407 240 B1), prior to destroying the mold itself in order to reveal the weld and proceed with finishing of the weld. Thereafter, the refractory felt is changed and the cap is ready for being used again on a new weld.
Nevertheless, this possibility of reusing the cap is limited insofar as the chimney becomes warped due to the temperatures to which it is subjected during the thermite reaction, and as a result it becomes progressively less suitable for standing in continuous manner on the rim of a cover, so that it starts to allow non-filtered fumes to escape between itself and the cover, and it no longer enables the filter to be positioned in repeatable manner and to be compressed between the grids, so the quality of filtering deteriorates with increasing number of uses.
In addition, the filter cap as presently made is quite heavy, weighing about 7 decanewtons (daN) to 8 daN which makes it awkward to transport from worksite to worksite, and this runs the risk of leading to cracks in the cover when it is made of agglomerated sand, as it is preferred, unless the cap is put onto the cover with a certain amount of care, and also runs the same risk of cracking the crucible when the assembly constituted by the cap and the cover is put into place thereon.
Furthermore, given the bulky nature of the cap in the vertical direction, its center of gravity is rather high and the position of the center of gravity of the assembly constituted by the crucible, its cover, and the filter cap is particularly high compared with the support given by the mold to the crucible. This drawback is particularly awkward insofar as the cover itself is quite heavy, weighing about 2.1 daN when it is made of agglomerated sand, and it too occupies a certain height, and consequently has a relatively high center of gravity, because of the disposition selected for the vent means in order to avoid as much as possible any particles being projected out from the crucible during the thermite reaction. This leads to a certain amount of instability firstly for the assembly constituted by the cover and its filter cap on the crucible, and secondly for the assembly constituted by the crucible, its cover, and its filter cap on the mold, when the crucible is supported directly by the mold as described in EP 0 407 240 B1, which means that great care needs to be taken in use, e.g. when welding rails that are canted.
The filter cap presently in use also presents the drawback of requiring a used filter and a new filter to be handled between two welding operations, which is disagreeable for welding personnel, given the nature of the materials constituting the filters. Furthermore, there is always the possibility of filter replacement being forgotten between two welding operations, i.e. of a filter being reused after it has already been partially clogged by the dust conveyed by the smoke or by particles projected through the vents of the cover, and that leads to a danger of the pressure of the gas given off by the thermite reaction lifting the filter cap, thereby making it largely ineffective, or even expelling it, thereby constituting a hazard for the welding personnel.
The same drawbacks are to be found if the metal produced by the thermite reaction in the crucible chamber is intended for applications other than welding, and the object of the present invention is to remedy at least some of those drawbacks, and in preferred implementations all of those drawbacks, independently of the intended application.